Although the present invention is applicable to other optical devices, the invention will be described below with respect to a riflescope embodiment. It is also to be understood that reference to the front of the apparatus means the objective lens end, while the rear of the apparatus is understood as being the ocular lens end.
Since the advent of firearms, ways and means have been sought to accurately aim these devices at targets. Evolving from simple notch-and-bead arrangements, the optical riflescope has proved to be the most effective device in fulfilling this task.
Riflescopes have been commercially available for many years and have remained largely unchanged since their introduction. These instruments comprise two basic types. The simplest form comprises a body tube, objective and ocular lenses disposed at essentially opposite ends of the tube, and an erector lens assembly in the body tube in the space between the objective and ocular lenses. Without the erector optics, the image formed by the objective lens would be seen inverted as in celestial telescopes. The inversion of images of celestial objects is of no consequence, but telescopes adapted for use in the aiming of firearms require the inclusion of erector optics to provide an image which appears upright. The erector optics are mounted within a secondary tube which is mounted within the main body tube of the riflescope. The erector optics tube is usually controllably pivotable by means of adjustment screws for the positional fine adjustment of the target image relative to an essentially stationary sighting reference. This reference usually consists of a pair of aiming crosshairs or a reticle disposed in the riflescope either at the focal plane of the objective, in front of the erector lens assembly, or at the rear plane of focus shared by the erector and the ocular optical elements.
In a second and somewhat more complex form of riflescope, the erector optics are also controllably movable along the optical axis within their tube in such a manner as to cause the focal length of the optical system to vary to provide an image of variable size. A ring external of the body tube is manipulated by the shooter to alter, at will, the size of the image formed by the riflescope within limits afforded by the design and structure of the particular device. This operation is typically called "zooming", and a range from three-to nine-power is common. At three-power, the image formed by the device is three times life-size, while at nine power it is nine times life-size.
The body tube, or housing of the riflescope, is mounted to the firearm. Once mounted, the riflescope is "zeroed." This operation establishes a precise relationship between the bore line of the firearm and the operator's visual line of sight through the riflescope. This adjustment is facilitated by the previously mentioned screw means constituting a horizontal (windage) adjustment and a vertical (elevation) adjustment. "Zeroing" is done in relation to an impact point at a known distance down range. For instance, a firearm may be "zeroed" so that the aiming reference, the reticle or crosshair intersection, is aligned with a point one hundred yards distant and is coincident with the placement or location of the projectile at that range. Once established, the alignment is permanent, unless the rigors of field use necessitate realignment by further zeroing.
In riflescopes of basic construction, the range to targets must be compensated for entirely by the shooter who first estimates a target's range and then imposes a degree of "holdover" estimated to compensate for projectile drop at that distance. However, few shooters become adept at estimating range and in compensating properly for bullet drop. In the field, variations in terrain, air density, and elevation can cause even the best of shooters to misjudge target range a significant percentage of the time, resulting in their often being well off the mark when firing. Shooters who are expert at such judgment are able to attain accuracy with only one or two specific cartridges and rifles with which they have become familiar by practice. With increasing target distance, the expert shooter aims ever higher in estimated increments of the target's image size: quarter height of target over, half height over, full height over, and so forth. This judgment is based directly on the size of the target image in the riflescope and, stated in the simplest possible way, the shooter knows that as down range distance increases, target image size decreases.
Limited static and dynamic means have evolved for range determination and aiming compensation based on these experiences.